1. The present invention relates to a novel hydrophone for the detection of sonic waves in water and to the formation of an array of such hydrophones for determining the direction of such sonic waves. The invention employs laser techniques and in particular the phase modulation of coherent illumination which occurs when light is scattered by particles vibrating under the influence of sonic wave motion.
2. Description of the Prior Art
It is known that waves are modulated when the path length between the source and a sensor is changing. The "Doppler" frequency shift, which is one such form of modulation, is a well known pnenomenon. For instance, it is the apparent change in frequency of a sonic wave when the source of the sonic wave is on a vehicle moving with respect to a listener. While the "Doppler" frequency shift was first known in respect to sonic phenomena, it is also known in respect to electromagnetic waves. In radio waves, for instance, a frequency shift also takes place when either an object reflecting waves back to a source, or the source of such waves is in motion. A typical circumstance in radar systems is one wherein a radar transmitter is located at a fixed ground location, while a wave reflecting object is a moving aircraft. If the radar wave reflected back to the transmitter is synchronously detected in such a way as to detect small differences in frequency from the transmitted wave, one will observe relatively low frequency modulations, which can be used to measure the speed of the aircraft.
While the Doppler frequency shift has been well known and widely used in respect to radar systems using radio frequency waves, its applications to light waves has been much more recent. It has never been doubted that such phenomena, which are applicable to radio waves, would also apply to light since both are electromagnetic waves. The effects were often confirmed by astronomical observations. To bring Doppler effects to practical use in respect to light there has been a rapid growth of light sources and of light processing technology. Laser light sources now exist of high absolute intensities, and of sufficiently high spectral purity and stability so that relatively small motion induced effects may be observed. With the advent of the laser has come a number of associated light detection techniques which have made possible a very precise examination of the light wave. The advent of photodetectors of high bandwidth has made it possible to heterodyne a light wave against itself and examine a relatively large range of shifts in frequency. The Bragg modulator, for instance, has made it possible to shift the frequency of the light wave by a fixed amount, typically from tens to hundreds of megacycles. Upon detection in an optical heterodyne system, the Bragg frequency shift has been used to provide a convenient carrier for amplifying and filtering any Doppler shifted light modulation terms.
The foregoing techniques for sensing Doppler frequency shifts have been applied to measure the speed of rotating machinery, aircraft velocities, the speed of airborne particulate matter, and the velocity of fluids containing seed particles.
The present invention, while also directed to the detection of motion induced modulation of light, is directed to motions on a much smaller scale, typically measured in angstrom or micron units; to motions which are vibratory in nature rather than simple velocities; and which small motions produce only a small recurring phase shift of the light rather than a long term frequency shift. In particular, the invention is applied to the detection and location of sonic waves in water.
In sonic wave detection, classical listening devices use piezoelectric or magneto-electric acoustic waves and electronic amplification to achieve very high sensitivities. Such sensitivities, however, must generally go unused since in the more important applications, such as to seagoing applications, the ambient levels of noise caused by sea animals and wave motion are many times higher than the thermal noise limits. When directionality is sought, piezoelectric devices are employed in large arrays. In general, piezoelectric arrays are not directional until the dimensions of the array become large in relation to the sonic wavelengths of interest. Large arrays are inconvenient, however, since in shipboard configurations the most desirable location is in the bow below the water line where the drag from a large array would be intolerable. In practice, it is not possible to make such an array directional at the low sonic frequency. It is the low sonic waves which are propagated with least absorption and where marine propulsion equipment produces the greatest sonic outputs. These low sonic frequencies normally lie in the range of from 10 to 200 hertz with the frequencies of from 50-100 hertz being one band of appreciable interest.